The wavelength of a tunable diode laser, e.g. a lead salt laser, is tuned by changing the temperature of the diode which, in turn, changes the refractive index of the diode material and thus the oscillation wavelength. Ordinarily, when this is done at frequencies above a few hertz, the heating is accomplished by changing the drive current to the diode. However, changing the drive current also varies the gain and hence the power output of the diode. In many applications, this is undesirable. For example, if such a laser is used in absorption spectrocscopy to measure the absorption of light by a trace gas, such a change in power output cause the beam modulation to include first and second harmonic signals that are unrelated to, but mimic, the absorption of the light by the trace gas.
One way to solve the above problem is to control both the wavelength and the power output of the laser as is done with some microwave sweep oscillators. Thus, it is known from the publication "Power Level Controlled Optical Sweep Oscillator Using A GaAs Semiconductor Laser" by S. Yamaguchi and M. Suzuki, IEEE Transactions On Instrumentation And Measurement, Vol. IM-36, pp. 789-796 (1977), to tune a laser by varying the laser current, while achieving power control by changing the temperature of the entire laser diode and its mounting structure. However, this approach is limited to sub-Hertz tuning because of the slowness of the temperature control.
It is also known from the publication "Photothermal Wavelength Modulation Of A Diode Laser" by C. Klimcak and J. Camparo, J. Opt. Soc. Am. B, Vol. 5, pp. 211-214 (1988), to use an external laser as a means of changing the temperature of a diode laser for purposes of tuning the diode laser. It is noted in that publication that this causes a much smaller change in power output than would ordinarily occur if the diode laser were tuned by varying its current. However, there is no teaching in that article of any form of active control of the power of the diode laser.